Reverse flow cooling in rotating electrical machines delivers cold gas simultaneously to both the stator and the rotor by providing parallel inlet paths to the stator core and the rotor entrance. As illustrated in the application drawings, cold gas flows from a cooler into the region in back of the stator core, which may contain baffles to control the air flow through the stator core, and into the air gap. Cold gas also flows from the cooler into the ends of the rotor, and, generally through cooling gas subslots and radially outwardly through gas flow passages in the rotor slots, to the air gap. A fan attached to the rotor circulates the exhaust gas from the air gap back to the cooler. The discharge of the cooling flow is into the air gap and out the annular exits at the ends of the air gap. At the stator end regions, the cooling gas flowpath is past the core end, flange, flux shield if used, endturns and to the fan entrance. Air and hydrogen are commonly used as the cooling gas.
Previous machines with reverse flow ventilation were large hydrogen generators with large air gaps between the rotor and the stator. The large air gap helped to alleviate the pressure gradient along the air gap. Moreover, the flow distribution through the core is less than uniform, only being controlled by baffling at the back of the core which added to the machine flow resistance. The rotor fans were of the axial setout type, and the fan performance was increased by twisted preturning vanes. However, preturning vanes add to the expense and complexity of the machine.
The ventilation system for a reverse flow generator with a narrow air gap inherently has a high pressure drop in the air gap between the rotor and the stator core. Analysis of these machines indicated a hot spot at the axial center line of the machine. Larger and longer machines lead to even higher pressures. This is because reverse flow ventilation discharges both the rotor and stator cooling flow into the air gap and all the gas must exit the limited annular area at the ends of the gap between the retaining ring and the stator core end iron. The high pressure drop in the air gap requires higher fan performance to maintain adequate stator core flows. This results in increased fan windage losses and lower machine efficiency. In addition the large air gap pressure drop causes a lack of stator cooling flow at the center of the generator. This is where the air gap pressure is the highest and the differential pressure across the core is the lowest.
The heat generating components at the stator end region include the end packets, the flange, conductor bars, and the electrical connections and buswork that lead to the machine terminals. A flux shield, when used, constitutes another heat generating component. With a fixed pressure head to drive the flow, if the end region components are cooled entirely with parallel flowstreams, the flow volume is maximized, resulting in increased flow losses and decreased machine efficiency. If all the components are cooled in series, the downstream components will be significantly hotter than those upstream and result in a system that generally requires an increased pressure head and has decreased machine efficiency.